CN114498557A - Self-adaptive out-of-region CT saturation discrimination method suitable for bus-tie CT disconnection condition - Google Patents

Abstract

The self-adaptive CT saturation judging method is suitable for the bus-tie CT disconnection condition, and is used for collecting the bus voltage and the current of each branch circuit connected with the bus; calculating sampling point differential current and braking current; and judging whether the bus-tie CT is broken or not. If the bus-tie CT is not disconnected, identifying the saturation of the outside fault CT through the sequence of sudden change of sampling points of the differential current and the differential braking current, and locking the differential protection of the fault bus section; if the bus-tie CT is disconnected, whether an out-of-area fault occurs is identified through the sequence of sudden change of sampling points of the large-difference differential current and the large-difference braking current; and determining the bus section needing to be locked through the sampling point change rate of the small-difference differential current of the two bus bars connected in the bus-tie at the fault moment. The bus main wiring method is suitable for bus main wiring modes with bus connection, such as double buses, double bus double sections, double bus single sections and the like.

Description

Self-adaptive out-of-region CT saturation discrimination method suitable for bus-tie CT disconnection condition

Technical Field

The invention belongs to the field of power systems, and particularly relates to a self-adaptive out-of-region CT saturation discrimination method suitable for a bus tie CT disconnection condition.

Background

Because the bus plays a role in collecting and distributing electric energy in an electric power system, bus protection plays a very important role in the field of relay protection of the electric power system. As the capacity of the power system becomes larger, the loss due to the stable destruction of the system becomes larger. The bus protection action will cause the power failure of a larger area. If the bus fault of the junction substation cannot be timely removed, the accident that the power system is stably damaged can be caused. Therefore, the bus protection device has reliable action and good performance, can quickly detect the position of the bus fault and timely and selectively remove the fault, and has very important significance for keeping the stability of the power system.

When a branch on the bus fails, the currents of all the branches on the bus flow to the failure point, which may cause the failed branch to be severely saturated by CT. In order to prevent bus protection maloperation caused by bus outside fault CT saturation, the outside fault CT saturation is generally identified by judging the waveform characteristics of small difference current and braking current, and differential motion is locked. However, under an abnormal operation condition of bus-bar CT disconnection, bus differential current and brake current are no longer reliable, and an automatic switching algorithm is needed to ensure that bus protection cannot be mistakenly operated if an out-of-area fault CT saturation occurs under the abnormal operation condition.

Disclosure of Invention

In order to solve the defects in the prior art, the invention aims to provide a self-adaptive out-of-region CT saturation discrimination method suitable for the bus tie CT disconnection condition.

The invention adopts the following technical scheme.

The self-adaptive out-of-region CT saturation discrimination method suitable for the bus tie CT disconnection condition comprises the following steps:

step 1, collecting bus voltage and each branch current connected with a bus;

step 2, calculating sampling point differential current and braking current, and judging whether the bus-tie CT is disconnected; and if the bus tie CT is not disconnected, executing the step 3, otherwise, executing the step 4.

Step 3, calculating a small-difference differential current break variable and a small-difference braking current break variable, judging whether the CT saturation of the external fault exists according to the sequence of the small-difference differential current and small-difference braking current sampling point break, if so, locking the differential protection of the fault bus section, otherwise, returning to the step 1;

step 4, identifying whether the CT saturation of the out-of-area fault occurs or not through the sequence of the sampling points of the large-difference differential current and the large-difference braking current, if so, executing the step 5, otherwise, returning to the step 1;

and 5, determining the bus section needing to be locked through the sampling point change rate of the small-difference differential current of the two bus bars connected in the bus-tie at the fault moment.

The differential current comprises large-difference differential current and small-difference differential current, wherein the large-difference differential current is the sum of currents of all branches except the bus-coupled branch on the two sections of buses; the small-difference differential current of each section of bus is the sum of the currents of all the branches on the section of bus;

the braking current comprises large difference braking current and small difference braking current, wherein the large difference braking current is the sum of absolute values of all branch currents except for the bus-coupled branch on the two sections of buses; the small-difference differential current of each section of bus is the sum of absolute values of all branch currents on the section of bus;

the large and small differential currents are calculated as follows:

the large difference braking current and the small difference braking current are calculated in the following mode:

in the formula (I), the compound is shown in the specification,

I_dis a differential current;

I_fis a braking current;

I_iis the ith branch current;

n is the number of branches.

And when all the following criteria are met, judging disconnection of the bus-tie CT after set time delay:

1) zero-sequence current appears on the bus tie;

2) the current of a certain phase is 0, and the large-difference differential current of the phase is balanced; the differential current of the two buses connected with the phase is unbalanced, and the sum of the differential current of the two buses connected with the phase is balanced.

The small-difference differential current break variable and the large-difference differential current break variable are calculated in the same way, and the calculation formula is as follows:

Δi_d＝i_d(t)-i_d(t-T)

in the formula (I), the compound is shown in the specification,

Δi_dis a differential current abrupt change;

i_dis the bus differential current;

t is the current moment;

t is a power frequency period: 0.02 s.

The small difference braking current sudden change amount and the large difference braking current sudden change amount have the same calculation mode, and the calculation formula is as follows:

Δi_f＝i_f(t)-i_f(t-T)

in the formula (I), the compound is shown in the specification,

Δi_fis the brake current break variable;

i_fis a small difference braking current;

t is the current time;

t is a power frequency period: 0.02 s.

The method for judging whether the CT is saturated due to the out-of-area fault under the condition that the bus-tie CT is not disconnected comprises the following steps:

s301, respectively counting the mutation times M of the small-difference differential current and the mutation times N of the small-difference braking current of each bus in the set time, wherein the mutation quantity delta i of the small-difference differential current_d>Iset1, which is recorded as a small-difference differential current sudden change of the bus section, wherein Iset1 is a set value of the small-difference differential current; if the sudden change amount delta i of the braking current is small difference_f>Iset2, recording that a sudden change of the small difference braking current occurs in the bus section, wherein Iset2 is a set value of the small difference braking current;

s302, if N-M is more than or equal to Z1 for any bus, judging that the bus outside-zone fault CT is saturated, and locking the bus differential protection; where Z1 is a constant and the result of Z1 times the sample interval time is greater than 2.5 ms.

The method for identifying whether the CT saturation of the out-of-area fault occurs under the condition of bus tie CT disconnection comprises the following steps:

step S401, counting the mutation times Q of the large-difference differential current and the mutation times S of the large-difference braking current; wherein the amount of abrupt change Δ I of the large-difference differential current_d>Iset3, recording as the occurrence of a sudden change of the large-difference differential current, wherein Iset3 is a set value of the large-difference differential current; sudden change delta I of large-difference braking current_f>Iset4, recording that the large-difference braking current has one sudden change, wherein Iset4 is a set value of the large-difference braking current;

and S402, within the set time, if S-Q is larger than or equal to Z, judging that the CT saturation of the out-of-range fault occurs, wherein Z1 is a constant, and Z1 times the sampling interval to be larger than 2.5 ms.

Within set time, the number of sampling points of a bus I meeting the sudden change of the differential current change rate is W, the number of sampling points of a bus II meeting the sudden change of the differential current change rate is U, if W is larger than or equal to Z2, and U is smaller than Z2, and Z2 is a constant, the bus I outside the area is considered to be in fault CT saturation, and the bus I differential motion is locked; if U is larger than or equal to Z2 and W is smaller than Z2, the CT outside the II bus area is considered to be saturated, and the differential motion of the II bus is locked.

The small difference differential current change rate mutation condition is as follows:

Δk_d1>Δk_d2

wherein, Δ k_d1The ratio of the change rate of the current sampling point to the change rate of the previous sampling point of the section of bus differential current is shown;

Δk_d2the ratio of the change rate of the previous sampling point of the small-difference differential current of a section of the bus to the change rates of the previous two sampling points is obtained;

Δk_d1＝((i_d(t)-i_d(t-Δt))/(i_d(t-Δt)-i_d(t-2Δt))

Δk_d2＝((i_d(t-Δt)-i_d(t-2Δt))/(i_d(t-2Δt)-i_d(t-3Δt))

in the formula (I), the compound is shown in the specification,

Δ t is the sampling interval time;

i_dis the bus differential current.

The value ranges of the small difference differential current set value Iset1, the small difference braking current set value Iset2, the large difference differential current set value Iset3 and the large difference braking current set value Iset4 are differential constant values which are 0.4-0.6 times.

The method has the advantages that compared with the prior art, the method can automatically switch criteria under the conditions that the bus with the bus link has external fault CT saturation and the bus link CT is abnormally disconnected, even if the small differential current and the small differential braking current are unreliable, the external fault CT saturation of the bus can still be judged through the waveform characteristics of the large differential current and the large differential braking current and the change rate of sampling points of the small differential current of two buses connected with the bus link, the differential protection is locked according to the bus section, the bus protection misoperation is avoided, the reliability of the bus protection action is improved, the protection misoperation is avoided, and the stability of a power system is kept.

Drawings

FIG. 1 is a flowchart of an adaptive out-of-region CT saturation discrimination method applicable to a bus tie CT disconnection condition according to the present invention;

fig. 2 is a schematic diagram of a double-bus fault branch according to an embodiment of the present invention.

Labeled as:

PT 1-voltage transformer 1 of the I bus;

a voltage transformer 2 of a PT2-II bus;

l and 2200-circuit breakers;

a CT-current transformer;

2G and 1G-disconnecting knife-switches of circuit breakers.

Detailed Description

The present application is further described below with reference to the accompanying drawings. The following examples are only for illustrating the technical solutions of the present invention more clearly, and the protection scope of the present application is not limited thereby.

The self-adaptive out-of-region CT saturation discrimination method suitable for the bus tie CT disconnection condition comprises the following steps:

step 1, collecting bus voltage and each branch current connected with a bus;

step 2, calculating sampling point differential current and braking current, and judging whether the bus-tie CT is disconnected; and if the bus tie CT is not disconnected, executing the step 3, otherwise, executing the step 4.

Step 3, calculating a small-difference differential current break variable and a small-difference braking current break variable, judging whether the CT saturation of the external fault exists according to the sequence of the small-difference differential current and small-difference braking current sampling point break, if so, locking the differential protection of the fault bus section, otherwise, returning to the step 1;

step 4, identifying whether the CT saturation of the out-of-area fault occurs or not through the sequence of the sampling points of the large-difference differential current and the large-difference braking current, if so, executing the step 5, otherwise, returning to the step 1;

and 5, determining the bus section needing to be locked through the sampling point change rate of the small-difference differential current of the two bus bars connected in the bus-tie at the fault moment.

The method is suitable for bus main wiring modes with bus coupling, such as double buses, double bus double sections, double bus single sections and the like, the embodiment takes double buses as an example, as shown in a flow chart of a self-adaptive CT saturation judging method suitable for the bus coupling CT disconnection condition in fig. 1, the embodiment comprises the following steps:

step 1, collecting bus voltage through a voltage transformer, and collecting each branch current connected with a bus through a current transformer;

taking a double bus as an example, as shown in the schematic diagram of the double-bus fault branch circuit in fig. 2, collecting secondary values of PT1 and PT2 on two segments of buses, that is, voltages of the two segments of buses; and collecting secondary values of the current transformers CT on the two sections of buses, namely the current of each branch.

Step 2, calculating sampling point differential current and braking current, judging whether the bus-tie CT is disconnected or not, and executing step 3 if the bus-tie CT is not disconnected; if the bus tie CT is disconnected, executing the step 4;

specifically, the differential currents include a large-difference differential current and a small-difference differential current; the braking current includes a large differential braking current and a small differential braking current. The large-difference differential current is the sum of currents of all branches except the bus-coupled branch on the two sections of buses, and the large-difference braking current is the sum of absolute values of currents of all branches except the bus-coupled branch on the two sections of buses; the bus differential current I is the sum of the currents of all the branches on the bus I, and the bus differential braking current I is the sum of the absolute values of the currents of all the branches on the bus I; the bus II differential current is the sum of the currents of all the branches on the bus II, and the bus II differential braking current is the sum of the absolute values of the currents of all the branches on the bus II. Differential current I_dAnd a braking current I_fThe calculation formula is as follows:

in the formula (I), the compound is shown in the specification,

I_ifor the ith branch current of the bus；

And n is the number of branches connected with the bus.

The differential and braking currents for both large and small differences are calculated according to the above equations. Whether the bus-tie CT is broken can be judged through the large-difference differential current, the I bus small-difference differential current and the II bus small-difference differential current.

When all the following criteria are met, the disconnection of the bus-tie CT is judged after the set time delay:

1) zero-sequence current appears on the bus tie;

2) the current of a certain phase is 0, and the large-difference differential current of the phase is balanced; the differential current of the two buses connected with the phase is unbalanced, and the sum of the differential current of the two buses connected with the phase is balanced.

Specifically, taking bus tie A broken line as an example,

bus tie CT disconnection criterion 1: zero-sequence current appears on the bus tie;

bus tie CT disconnection criterion 2: the current of the bus-tie A is 0, and the large-difference differential current of the phase A is balanced; the differential current of the A phase of the bus I and the differential current of the A phase of the bus II are unbalanced, and the sum of the differential currents of the I bus and the II bus of the A phase is balanced.

And the two criteria are met simultaneously, and the bus-tie A-phase CT disconnection is judged through fixed time delay. The criterion of disconnection of the B phase and the C phase is the same as that of the A phase.

And (4) if the bus tie is not disconnected by the CT, executing the step 3, and if the bus tie is disconnected by the CT, executing the step 4.

And step 3: judging whether the CT saturation of the out-of-area fault occurs or not through the sequence of the sampling points of the small-difference differential current and the small-difference braking current, if so, locking the differential protection of the corresponding bus section, otherwise, returning to the step 1;

when CT is saturated, the magnetic flux of the iron core of the current transformer cannot change suddenly, when a fault starts and a primary current crosses zero from the reverse direction, the current transformer has a linear transfer area, a secondary current and the primary current meet the linear relation, and the size of the linear transfer area is related to the saturation degree of the current transformer. When the bus outside-zone fault CT is saturated, the differential current is 0 in the linear zone, and the braking current suddenly changes. This feature can therefore be used to identify out-of-range faults.

The small-difference differential current break variable and the large-difference differential current break variable are calculated in the same way, and the calculation formula is as follows:

Δi_d＝i_d(t)-i_d(t-T)

in the formula (I), the compound is shown in the specification,

Δi_dis a differential current abrupt change;

i_dis the bus differential current;

t is the current time;

t is a power frequency period: 0.02 s.

The small difference braking current sudden change amount and the large difference braking current sudden change amount have the same calculation mode, and the calculation formula is as follows:

Δi_f＝i_f(t)-i_f(t-T)

in the formula (I), the compound is shown in the specification,

Δi_fis the sudden amount of braking current;

i_fis a small difference braking current;

t is the current time;

t is a power frequency period: 0.02 s.

Taking double bus as an example, the small difference differential current abrupt change amount Delta i of the I bus_d1The algorithm is as follows:

Δi_d1＝i_d1(t)-i_d1(t-T)

in the formula:

t is the current time;

t is a power frequency period: 0.02 s;

i_d1is I bus small difference differential current;

small difference braking current abrupt change delta i of I bus_f1The algorithm is as follows:

Δi_f1＝i_f1(t)-i_f1(t-T)

in the formula (I), the compound is shown in the specification,

t is the current time;

t is a power frequency period: 0.02 s;

i_f1is a I motherLine differential braking current.

And counting the mutation times of the differential current of the I bus and the mutation times of the braking current of the bus by adopting a sampling point data window. I bus small difference differential current sudden change delta i_d1>Iset1, noted as 1, otherwise noted as 0, where Iset1 is the I bus differential current setpoint. I sudden change delta i of bus differential braking current_f1>Iset2, noted as 1, otherwise noted as 0, where Iset1 is the I bus differential braking current set point. Preferably, the value range of the set value of the I bus small-difference differential current is 0.4-0.6 times of the differential constant value; the value range of the set value of the I bus small-difference braking current is 0.4-0.6 times of the differential constant value.

Within the set time, the counted number of the sudden changes of the I bus differential current meeting the conditions is M, the number of the sudden changes of the I bus differential braking current meeting the conditions is N, if N-M is larger than or equal to Z, namely the I bus differential braking current is suddenly changed before the I bus differential current, the I bus out-of-area fault CT saturation is considered to occur, and I bus differential protection is locked. The criterion of the second bus is the same as that of the first bus. Z is a constant and in a preferred but non-limiting embodiment of the invention, the result of Z multiplied by the sampling interval time is greater than 2.5 ms.

Step 4, identifying whether the CT saturation of the out-of-area fault occurs or not through the sequence of the sudden change of the sampling points of the large-difference differential current and the large-difference braking current, and if so, executing the step 5; otherwise, returning to the step 1

The large-difference brake current of the I bus is firstly suddenly changed compared with the large-difference differential current of the I bus, when the bus-bar CT is disconnected, the CT saturation criterion of the external fault branch cannot be applied to the scene of the step when the CT saturation of the external fault branch occurs in a linear region, the small-difference differential current and the small-difference brake current are simultaneously suddenly changed, therefore, whether the external fault occurs or not is identified by judging the sudden change sequence of the large-difference differential current and the large-difference brake current, and then the locked bus section is determined by judging the sampling point change rate of the small-difference differential current and the small-difference brake current at the fault moment.

The big difference differential current and big difference brake current sampling point mutation criterion is similar to the step 3, and specifically:

large differential current abrupt change Δ i_dThe algorithm is as follows:

Δi_d＝i_d(t)-i_d(t-T)

t is the current time;

t is a power frequency period: 0.02 s;

i_dis a large differential current;

large difference braking current abrupt change delta i_fThe algorithm is as follows:

Δi_f＝i_f(t)-i_f(t-T)

in the formula (I), the compound is shown in the specification,

t is the current moment;

t is a power frequency period: 0.02 s;

i_fis a large differential braking current.

And counting the mutation times of the large-difference differential current and the mutation quantity times of the large-difference braking current by adopting a sampling point data window. At any sampling point time, if the difference is large, the sudden change quantity delta i of the differential current_d>Iset3, recording as the occurrence of a large-difference differential current sudden change, wherein Iset3 is a large-difference differential current set value; if the sudden change amount delta i of the braking current is large difference_f>Iset4, which is recorded as the occurrence of a sudden change in the differential braking current, wherein Iset4 is the set point of the differential braking current.

Preferably, the value range of the large-difference differential current set value Iset3 is 0.4-0.6 times of the differential constant value; the value range of the large-difference braking current set value Iset4 is a differential constant value which is 0.4-0.6 times.

Specifically, the differential constant value is a standard value calculated according to the relay protection setting procedure and the actual power grid parameter.

And in the set time, counting the mutation times of the large-difference differential current meeting the conditions to be Q and the mutation times of the large-difference brake current meeting the conditions to be S, if S-Q is more than or equal to Z1, namely the large-difference brake current mutates earlier than the large-difference differential current, considering that the CT of the out-of-zone fault is saturated and executing the step 5, otherwise, executing the step 1. Z1 is constant and preferably the result of Z1 times the sampling interval is greater than 2.5 ms.

And 5, determining the bus section needing to be locked according to the sampling point change rate of the small-difference differential current of the two sections of buses at the fault moment.

If one branch CT is saturated, the small-difference differential current of the bus I and the bus II is a power frequency quantity sine wave in a saturated linear region when the fault starts; when the current enters a saturated nonlinear region, the differential current of the bus connected with the CT saturated branch circuit is distorted, and the differential current of the other section of bus is still a power frequency sine wave. According to the waveform characteristics of two buses when the CT saturation occurs in the external fault, the bus section needing to be locked can be selected by calculating the change rate of the differential current of the bus I and the bus II.

Within set time, the number of sampling points of a bus I meeting the sudden change of the differential current change rate is W, the number of sampling points of a bus II meeting the sudden change of the differential current change rate is U, if W is larger than or equal to Z2, and U is smaller than Z2, and Z2 is a constant, the bus I outside the area is considered to be in fault CT saturation, and the bus I differential motion is locked; if U is larger than or equal to Z2 and W is smaller than Z2, the CT outside the II bus area is considered to be saturated, and the differential motion of the II bus is locked.

The small-difference differential current change rate mutation conditions are as follows:

Δk_d1>Δk_d2

wherein, Δ k_d1The ratio of the change rate of the current sampling point to the change rate of the previous sampling point of the section of bus differential current is shown;

Δk_d2the ratio of the change rate of the previous sampling point of the small-difference differential current of a section of the bus to the change rates of the previous two sampling points is obtained;

Δk_d1＝((i_d(t)-i_d(t-Δt))/(i_d(t-Δt)-i_d(t-2Δt))

Δk_d2＝((i_d(t-Δt)-i_d(t-2Δt))/(i_d(t-2Δt)-i_d(t-3Δt))

in the formula (I), the compound is shown in the specification,

Δ t is the sampling interval time;

i_dis the bus differential current.

In this embodiment, an i bus is taken as an example.

Δk_d11＝((i_d1(t)-i_d1(t-Δt))/(i_d1(t-Δt)-i_d1(t-2Δt))

Δk_d12＝((i_d1(t-Δt)-i_d1(t-2Δt))/(i_d1(t-2Δt)-i_d1(t-3Δt))

Δk_d21＝((i_d2(t)-i_d2(t-Δt))/(i_d2(t-Δt)-i_d2(t-2Δt))

Δk_d22＝((i_d2(t-Δt)-i_d2(t-2Δt))/(i_d2(t-2Δt)-i_d2(t-3Δt))

In the formula (I), the compound is shown in the specification,

Δk_d11the ratio of the current point change rate of the bus differential current to the previous point change rate is I;

Δk_d12the ratio of the change rate of the previous point of the bus differential current to the change rates of the previous two points is I;

Δk_d21the ratio of the current point change rate to the previous point change rate of the bus differential current II is shown;

Δk_d22the ratio of the change rate of the previous point of the bus differential current to the change rates of the previous two points is II;

t is the current time;

Δ t is the sampling interval time;

i_d1the differential current of the I bus is small;

i_d2is the differential current of bus II.

And counting the small-difference differential current change condition of each section of bus by adopting a sampling point data window. Within a set time, the condition that delta k is satisfied is counted_d11>Δk_d12The number of sampling points of differential current of the I bus is W, and the requirement of delta k is met_d21>Δk_d22If W is more than or equal to Z2 and U is less than or equal to U<And Z2, the fault CT outside the I bus area is considered to be saturated, and the I bus differential is locked. Otherwise, returning to the step 1. Z2 is a constant, and Z2 preferably takes the value of 3.

The present applicant has described and illustrated embodiments of the present invention in detail with reference to the accompanying drawings, but it should be understood by those skilled in the art that the above embodiments are merely preferred embodiments of the present invention, and the detailed description is only for the purpose of helping the reader to better understand the spirit of the present invention, and not for limiting the scope of the present invention, and on the contrary, any improvement or modification made based on the spirit of the present invention should fall within the scope of the present invention.

Claims (10)

1. The self-adaptive out-of-region CT saturation discrimination method suitable for the bus tie CT disconnection condition is characterized by comprising the following steps of:

step 1, collecting bus voltage and each branch current connected with a bus;

step 2, calculating sampling point differential current and braking current, and judging whether the bus-tie CT is disconnected; and if the bus tie CT is not disconnected, executing the step 3, otherwise, executing the step 4.

Step 3, calculating a small-difference differential current break variable and a small-difference braking current break variable, judging whether the CT saturation of the external fault exists according to the sequence of the small-difference differential current and small-difference braking current sampling point break, if so, locking the differential protection of the fault bus section, otherwise, returning to the step 1;

step 4, identifying whether the CT saturation of the out-of-area fault occurs or not through the sequence of the sampling points of the large-difference differential current and the large-difference braking current, if so, executing the step 5, otherwise, returning to the step 1;

and 5, determining the bus section needing to be locked through the sampling point change rate of the small-difference differential current of the two bus bars connected in the bus-tie at the fault moment.

2. The adaptive out-of-region CT saturation discrimination method suitable for the bus tie CT disconnection condition as claimed in claim 1,

the differential current comprises a large-difference differential current and a small-difference differential current, wherein the large-difference differential current is the sum of currents of all branches except the bus-coupled branch on the two sections of buses; the small-difference differential current of each section of bus is the sum of the currents of all the branches on the section of bus;

the braking current comprises large difference braking current and small difference braking current, wherein the large difference braking current is the sum of absolute values of all branch currents except for the bus-coupled branch on the two sections of buses; the small-difference differential current of each section of bus is the sum of absolute values of all branch currents on the section of bus;

the large and small differential currents are calculated as follows:

the large difference braking current and the small difference braking current are calculated in the following mode:

in the formula (I), the compound is shown in the specification,

I_dis a differential current;

I_fis a braking current;

I_iis the ith branch current;

n is the number of branches.

3. The adaptive out-of-region CT saturation discrimination method suitable for the bus tie CT disconnection condition as claimed in claim 1,

and when all the following criteria are met, judging disconnection of the bus-tie CT after set time delay:

1) zero-sequence current appears on the bus tie;

2) the current of a certain phase is 0, and the large-difference differential current of the phase is balanced; the differential current of the two buses connected with the phase is unbalanced, and the sum of the differential current of the two buses connected with the phase is balanced.

4. The adaptive out-of-region CT saturation discrimination method suitable for the bus tie CT disconnection condition as claimed in claim 1,

the small-difference differential current break variable and the large-difference differential current break variable are calculated in the same way, and the calculation formula is as follows:

Δi_d＝i_d(t)-i_d(t-T)

in the formula (I), the compound is shown in the specification,

Δi_dis a differential current abrupt change;

i_dis the bus differential current;

t is the current time;

t is a power frequency period: 0.02 s.

5. The adaptive out-of-region CT saturation discrimination method suitable for the bus tie CT disconnection condition as claimed in claim 1,

the small difference braking current sudden change amount and the large difference braking current sudden change amount have the same calculation mode, and the calculation formula is as follows:

Δi_f＝i_f(t)-i_f(t-T)

in the formula (I), the compound is shown in the specification,

Δi_fis the brake current break variable;

i_fis a small difference braking current;

t is the current time;

t is a power frequency period: 0.02 s.

6. The adaptive out-of-region CT saturation discriminating method applied to the case of bus tie CT disconnection according to claim 2 or 3,

step 3, judging whether the CT saturation of the out-of-range fault exists or not comprises the following steps:

s301, respectively counting the mutation times M of the small-difference differential current and the mutation times N of the small-difference braking current of each bus in the set time, wherein the mutation quantity delta i of the small-difference differential current_d>Iset1, which is recorded as a small-difference differential current sudden change of the bus section, wherein Iset1 is a set value of the small-difference differential current; if the sudden change amount delta i of the braking current is small difference_f>Iset2, which is the occurrence of one small-difference braking electricity for the bus-sectionA current spike, where Iset2 is the small differential braking current set point;

s302, if N-M is more than or equal to Z1 for any bus, judging that the bus outside-zone fault CT is saturated, and locking the bus differential protection; where Z1 is a constant and the result of Z1 times the sample interval time is greater than 2.5 ms.

7. The adaptive out-of-region CT saturation discrimination method suitable for the bus tie CT disconnection condition as claimed in claim 1,

the step 4 of identifying whether the CT saturation of the out-of-area fault occurs comprises the following steps:

step S401, counting the mutation times Q of the large-difference differential current and the mutation times S of the large-difference braking current; wherein the amount of abrupt change Δ I of the large-difference differential current_d>Iset3, recording as the large-difference differential current has a sudden change, wherein Iset3 is a set value of the large-difference differential current; sudden change delta I of large-difference braking current_f>Iset4, recording that the large-difference braking current has one sudden change, wherein Iset4 is a set value of the large-difference braking current;

and S402, within the set time, if S-Q is larger than or equal to Z, judging that the CT saturation of the out-of-range fault occurs, wherein Z1 is a constant, and the multiplied sampling interval of Z1 is larger than 2.5 ms.

8. The adaptive out-of-region CT saturation discrimination method suitable for the bus tie CT disconnection condition as claimed in claim 1,

within set time, the number of sampling points of a bus I meeting the sudden change of the differential current change rate is W, the number of sampling points of a bus II meeting the sudden change of the differential current change rate is U, if W is larger than or equal to Z2, and U is smaller than Z2, and Z2 is a constant, the bus I outside the area is considered to be in fault CT saturation, and the bus I differential motion is locked; if U is larger than or equal to Z2 and W is smaller than Z2, the CT outside the II bus area is considered to be saturated, and the differential motion of the II bus is locked.

9. The adaptive out-of-region CT saturation discrimination method for bus-tie CT disconnection condition as claimed in claim 8,

the small difference differential current change rate mutation condition is as follows:

Δk_d1＞Δk_d2

wherein, Δ k_d1The ratio of the change rate of the current sampling point to the change rate of the previous sampling point of the section of bus differential current is shown;

Δk_d2the ratio of the change rate of the previous sampling point of the small-difference differential current of a section of the bus to the change rates of the previous two sampling points is obtained;

Δk_d1＝((i_d(t)-i_d(t-Δt))/(i_d(t-Δt)-i_d(t-2Δt))

Δk_d2＝((i_d(t-Δt)-i_d(t-2Δt))/(i_d(t-2Δt)-i_d(t-3Δt))

in the formula (I), the compound is shown in the specification,

Δ t is the sampling interval time;

i_dis the bus differential current.

10. The adaptive out-of-region CT saturation discriminating method applied to the case of bus tie CT disconnection according to claim 6 or 7,

the value ranges of the small difference differential current set value Iset1, the small difference braking current set value Iset2, the large difference differential current set value Iset3 and the large difference braking current set value Iset4 are differential constant values which are 0.4-0.6 times.

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